It has long been known that cellulose derivatives of relatively high molecular weight are readily depolymerized or degraded by treatment with strong mineral acid at moderate temperatures to produce corresponding polymers of much lower molecular weight. Cellulose derivatives such as lower alkyl and hydroxy lower alkyl ethers, for example, methyl cellulose, ethyl cellulose, hydroxypropyl cellulose, and methyl hydroxybutyl cellulose, cellulose esters such as the acetate and butyrate, and methyl hydroxylpropyl cellulose phthalate, carboxyalkyl cellulose such as carboxymethyl cellulose, and other such derivatives having two or more of such substituents on the cellulose molecule are representative examples of the class. The derivatives of relatively low molecular weight are of interest particularly because of their higher solubility in water which makes them useful as modifiers in detergent compositions and as water-dispersible films for coating medical tablets and like applications.
This kind of acid-promoted degradation or depolymerization is conveniently done by contacting the essentially dry powdered cellulosic polymer with gaseous HCl or other hydrogen halide in a fluidized bed operation, for example, or by contacting the powder or slurry of the powder in an inert organic liquid with gaseous halide in a mixing device such as a rotating mixer, ribbon blender, or the like. For the slurry type of depolymerization, a relatively low boiling inert and essentially anhydrous organic liquid with boiling point below 100.degree. C is preferred. Since the cellulosic polymers are essentially insoluble in common organic solvents, substantially any such solvent can be used, for example, methylene chloride, methanol, 1,1,1-trichloroethane, carbon tetrachloride, acetone, hexane, and benzene. In any of these modes of depolymerization, a moderate temperature from ambient temperature to about 80.degree. C is suitable.
Residual acid in the polymer product can be removed by purging the dry powder or a slurry of the powder with air or nitrogen or by washing the powder with an organic solvent such as described above. The last traces of acid can be removed by neutralization with a weak base. A dry, essentially pure product with no need for further processing for many applications is thereby provided.
Hydrogen halide initiated depolymerizations of the kind described are disclosed by Reid, U.S. Pat. No. 1,864,554 and by Ouno, U.S. Pat. No. 3,391,135.
Unfortunately, the low molecular weight polymers made by these processes typically develop a yellow or brown color and so are undesirable for many applications.
It is known that high molecular weight cellulose ethers and esters can be bleached or brightened by stirring a slurry of the cellulosic polymer in aqueous lower alkanol containing dissolved bisulfite ion. This process is described by Whitmeyer in U.S. Patent 3,549,617. Although the brightening treatment disclosed in that patent is effective for the higher molecular weight ethers and esters described therein which are insoluble in aqueous alkanol, the corresponding low molecular weight polymers produced by HCl-initiated depolymerization are much more affected by the presence of significant quantities of water and form unmanageable gels or actually dissolve when slurried in aqueous alkanol as described by Whitmeyer.